The Friday offer from President Obama's campaign to Mitt Romney — that if the GOP presidential candidate releases his tax returns for the past five years, it won't attack him for not releasing more — was immediately rejected by the Romney campaign.

But the give-and-take keeps Romney on the defensive, and promises to keep the issue of Romney's taxes going for weeks to come.

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Up next, math books get a makeover. You may remember my next guest from her acting roles on "The Wonder Years." Winnie Cooper may ring a bell or "The West Wing." But for thousands of girls today, she's the writer and personality behind a bestselling series of books that aim to teach girls about math. First, there was "Math Doesn't Suck," then "Hot X: Algebra Revealed," and now we're onto geometry.

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This is SCIENCE FRIDAY, and I'm Flora Lichtman. On any given day, visit the website brainpickings.org, and you'll find posts on topics ranging from Charles Darwin's notes on marriage, to a birthday tribute to Julia Child, to the poetry of Patti Smith.

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This is SCIENCE FRIDAY. I'm Flora Lichtman, filling in for Ira Flatow today. Do you know your blood type? You may have wondered about it this week if you heard news linking blood type to your risk of heart disease. In that study, researchers determined that those with blood type O had the lowest risk of heart disease and those with AB the highest.

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This is SCIENCE FRIDAY. I'm Flora Lichtman, filling in for Ira Flatow, who's out today. This week, the FDA approved a new influenza vaccine for this year's flu season, and soon enough summer will be over and you'll be standing in line again at your pharmacy or doctor's office, participating in that yearly ritual - your annual vaccination allocation.

The Reinvent the Toilet Challenge asked engineers to dream up a replacement for the antiquated flush toilet. Michael Hoffmann and his team at Caltech responded with a solar-powered toilet that disinfects waste and reuses wastewater to flush. Better yet, it pumps out hydrogen gas for use in fuel cells.

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This is SCIENCE FRIDAY. I'm Flora Lichtman. We're in the midst of the worst drought in over 50 years. Water tables are dropping faster than they can be replenished, and at the same time an op-ed in the New York Times today says that the United States is estimated to lose about one in six gallons, one in six gallons of clean water every day due to leaky pipes in the ground.